1. Field of the Invention
The invention relates generally to television transmission and reception. More particularly, the invention relates to television transmitting and receiving equipment and methods in which digitally encoded audio signals are compatibly carried with a standard NTSC color television signal, including environments in which the lower adjacent channel carries an NTSC color television signal in which the analog FM sound carrier is modulated in accordance with the BTSC multi-channel television sound system.
2. Description of Related Art
There is a growing interest in the use of digital data transmission techniques to improve broadcast audio quality. Digital transmission is inherently robust. While the coding of high quality audio into digital form theoretically entails a loss of quality, there is no further loss of quality when the digits are transmitted through an error-free channel. This is in stark contrast to the transmission of analog audio, in which a "perfect" channel is required to avoid degradation. Unfortunately, the intercarrier sound channel used with broadcast television inherently limits the sound quality of the analog BTSC (Broadcast Television Systems Committee) Multi-Channel Television Sound (MTS) System. The BTSC MTS system is described in "The BTSC Multi-Channel Television Sound System" by Carl G. Eilers, SMPTE Journal, November 1986, pp. 1134-1138 and "Multichannel Television Sound Broadcasting in the United States," by Randy Hoffner, J. Audio. Eng. Soc., Vol. 35, No. 9., 1987 September, pp. 660-665.
Digital transmission techniques have matured to the point that they can be applied to broadcast audio. Previous work by the present inventor led to a proposal for the addition of digital audio to the NTSC television broadcast signal. The proposal was published in two articles: "A Compatible Digital Audio Format for Broadcast and Cable Television" by Craig C. Todd, IEEE Trans on Consumer Electronics, Vol. CE-33, No 3, August 1987, pp. 297-305 and "Digital Sound and Data for Broadcast Television--A Compatible System," NAB Proceedings, 41st NAB Engineering Conference, 1987.
The inventor's 1987 proposal was based in part on work performed in Sweden and Finland in which a 512 kilobit per second (kb/s) Quadrature Phase Shift Keying (QPSK) carrier was extensively tested with PAL system B. See "Digital Multi-Channel Sound for Television," by Anders Nyberg, ICCE Digest, June 1987 and "Digital Two-Channel Sound for Terrestrial Television," ICCE Digest, June 1987. The similarities between the B-PAL television system and the M-NTSC television system with respect to signal component spectral locations suggested to the inventor that the Scandinavian test results would apply in the United States. "M-NTSC" is the designation for the standard 525 line/60 field per second NTSC system used in almost all countries using the NTSC system.
A similar digital audio system for television was also developed in the U.K. for the PAL system G. See "The U.K. System for Digital Stereo Sound with Terrestrial Television," by S.R. Ely, J Audio Eng. Soc., Vol. 35, No 9, 1987, pp. 652-659. The PAL B and G systems differ principally in that the spacing between adjacent channel video carriers is 7 MHz in the PAL B system and 8 MHz in the PAL G system.
The inventor's original 1987 proposal was as follows:
A. A modulated QPSK carrier having a nominal bandwidth of about 435 kHz. PA1 B. A digital audio carrier frequency 4.85 MHz above video carrier frequency (1.15 MHz below the video carrier frequency of the adjacent higher channel NTSC signal). PA1 C. A digital audio carrier level at -20 dB with respect to the peak vision carrier level.
In compatibility testing of the inventor's 1987 system in a clean laboratory setting, some television receiving sets exhibited interference to the NTSC signal in the upper adjacent video channel as a result of the presence of the QPSK modulated digital audio carrier. The interference affected the luminance signal component and manifested itself as additive noise between approximately 1 MHz and 1.4 MHz. The noise level in a television receiving set exhibiting such interference is subjectively similar to the noise level resulting from a video carrier-to-noise ratio (CNR) of approximately 47 dB. While this level of interference may not be detectable in view of the low quality level of signals in many consumers' homes, the potential of improved transmission and reception systems, such as optical fibre to raise the quality level of cable television, warrants a reduction in the level of interference by approximately 6 dB in order to insure compatibility in the future. In addition, the nature of existing broadcast transmitters also calls for some changes so that the signal is more easily transmitted.